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15 include "nat/plus.ma".
19 ((λx,y.x + S y = S (x + y)) ? ?) →
20 ((λx,y.y + x = x + y) ? ?) →
21 ∀x. ((λz. x + x = z + S z) ?).
22 ##[ #H; #H1; #H2; #x; nnormalize in H H1 H2 ⊢ %; nauto by H, H1, H2.